Remove TABLES_GIT variable
Replace pip virtualenv installation by message and script exit

Since python_deps.sh is now standalone, we do not know where where are in the
filesystem and do want to install a virtualenv anywhere on the disk. If no pip
is found in the PATH,  a warning message is displayed asking to install pip
and the script is terminated.
Also use pip directly rather than cd'ing to the parent directory and using
bin/pip.